There are porphyries which have been injected from below amongst strata, and others ejected, which have flowed in streams; it is remarkable, and I could show specimens of this rock produced in these three methods, which cannot be distinguished.

It is a great mistake considering the Cordilleras here as composed of rocks which have flowed in streams.

In this range I nowhere saw a fragment, which I believe to have thus originated, although the road passes at no great distance from the active volcanoes.

The porphyries, conglomerate, sandstone and quartzose sandstone and limestones alternate and pass into each other many times, overlying (where not broken through by the granite) clay-slate.

In the upper parts, the sandstone begins to alternate with gypsum, till at last we have this substance of a stupendous thickness.
I really think the formation is in some places (it varies much) nearly 2,000 feet thick, it occurs often with a green (epidote ?) siliceous sandstone and snow-white marble; it resembles that found in the Alps in containing large concretions of a crystalline marble of a blackish grey colour.